In many instances, machines such as for example electric motors, electric generators, internal-combustion engines, jet engines, turbines, and the like, and the systems they drive, are actively monitored by various monitoring systems for performance and operational characteristics including for example vibration, heat, noise, electrical characteristics (e.g., current, voltage, resistance, etc.), environmental effects, and the like. Generally, the monitoring systems that monitor these machines are comprised of one or more transducers that are proximate to and associated with the machine. For example, passive magnetic or reluctance sensors may be used by monitoring systems. Hereinafter, these sensors will be referred to simply as “sensors, “magnetic sensors,” or “passive magnetic sensors,” which is intended to include passive magnetic and reluctance sensors.
Generally, passive magnetic sensors are constructed of a permanent magnet and a coil with signal wires connecting to each side of the coil. The magnet creates a field (lines of flux), which extends from the end of the magnetic sensor into the air. As a ferrous object approaches the tip of the magnetic sensor (i.e., the probe tip), the object interacts with the magnetic field originating from the magnet encased in the magnetic sensor, thereby inducing a current flow in the coil and in turn creating alternating current (AC) voltage that can be seen on the signal wires acting as the magnetic sensor outputs. As the target enters and then leaves area occupied by the flux lines, this creates a positive voltage peak followed by a negative voltage peak. The voltage output can appear to be sinusoidal in nature, but can be distorted depending on the material composition and geometry of the target. Several factors contribute to the characteristics of the output signal generated by passive magnetic sensors including surface speed of the target, gap size, target geometry, and load impedance.
Surface speed is the speed at which the target passes the magnetic sensor's probe tip and directly affects the amplitude of the pulse created by the magnetic sensor. The exact function relating target speed to output voltage varies from magnetic sensor to magnetic sensor, but correlation between speed and output voltage is nearly a linear function (proportional). Gap size refers to the distance between the target and the magnetic sensor's probe tip when the target passes and it influences the output voltage as well. The smaller the gap, the larger the output voltage will be. Typical gap settings for magnetic sensors can be around 25 to 30 mils. Generally, the relationship between gap size and voltage output is nonlinear in nature. Decreasing the gap can drastically increase output voltage. The geometric dimensions of a target can also affect the amplitude and shape of the output voltage. Generally, the larger the target, the greater the amplitude. Load impedance, relative to the internal impedance of the magnetic sensor, dictates the amount of magnetic sensor output voltage that will be seen by that load. Magnetic sensors are generally designed with the lowest practical impedance consistent with providing maximum output. The load impedance should be high in relation to the impedance of the magnetic sensor to minimize the voltage drop across the coil and to deliver the maximum output to the load. Generally the load impedance should be at least 10 times that of the internal impedance of the magnetic sensor.
Benefits of using magnetic sensors include that they are passive and therefore don't require external power, they are simplistic in design and therefore highly reliable, and they are generally low cost. However, there are challenges to using these sensors as well. One challenge is that output signal amplitude can fluctuate drastically based on speed. This can make it difficult to analyze startup data characteristics of a machine ramping from, for example, zero to 3600 rpm. For example, magnetic sensor output voltage changes drastically (e.g., 10 mVp-p to 200Vp-p) with respect to the speed of the passing target. This characteristic can pose a challenge when monitoring the magnetic sensor output of a target passing at a low speed with the same accuracy of the output when the target passes at high speed. Slow speed signals need gain to improve the signal to noise ratio while high speed signals need to be attenuated to avoid clipping or distortion due to circuit limits. Another challenge is that due to the passive nature of magnetic sensors, a magnetic pickup has less drive strength and will not be able to drive a signal through long cables. Furthermore, magnetic sensors cannot be used for an accurate gap reading. While the amplitude can be indicative of the gap, the gap size cannot be accurately determined with a given output voltage due to the various other factors that influence the output as discussed above.
Therefore, systems and methods that overcome challenges in the art, some of which are described above, are desired. In particular, providing dynamic automatic gain control for a magnetic sensor that can be used to improve the accuracy of an output of the magnetic sensor for a target passing at a low speed and of the same target when the target passes at high speed would be valuable in addressing the above-described challenges.